1. Field of the Invention
The present invention relates to apparatus and method for drilling a well into earth formations lying below a body of water, wherein the wellhead equipment of the well is positioned below the surface of the water. The well is drilled from a floating drilling vessel, with a riser conduit connecting the vessel drilling equipment to the wellhead assembly.
2. Description of the Prior Art
An increasing amount of offshore deepwater exploratory well drilling is being conducted in an attempt to locate oil and gas reservoirs. These exploratory wells are generally drilled from floating vessels. As in any drilling operation, drilling fluid must be circulated through the drill bit in order to cool the bit and to carry away the cuttings. This drilling fluid is normally returned to the floating vessel by means of a large diameter pipe, known as a riser, which extends between the subsea wellhead assembly and the floating vessel. The lower end of this riser is connected to the wellhead assembly which is generally located adjacent the ocean floor, and the upper end usually extends through a centrally located hull opening of the floating vessel. A drill string extends downward through the riser into earth formations lying below the body of water, and drilling fluids circulate downwardly through the drill string, out through the drilling bit, and then upwardly through the annular space between the drill string and the riser, returning to the vessel.
As these drilling operations progress into deeper waters, the length of the riser and consequently its unsupported weight also increases. Since the riser has the same structural buckling characteristics as a vertical column, riser structural failure may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. Two separate mechanisms are typically used to avoid the possibility of this cause of riser failure.
Riser tensioning systems are installed onboard the vessel, which apply an upward force to the upper end of the riser, usually by means of cable, sheave, and pneumatic cylinder mechanisms connected between the vessel and the upper elements of the riser.
In addition, buoyancy or ballasting devices may also be attached to the submerged portion of the riser. These devices are usually comprised of syntactic foam or individual ballast or buoyancy tanks formed on the outer elements of the riser section. The ballast or buoyancy tanks are capable of being selectively inflated with air by utilization of the floating vessel's air compression equipment. Both of these buoyancy devices create upwardly directed forces in the riser, compensating for the compressive stresses created by the riser's weight, and thereby prevent riser failure.
Since the riser if fixedly secured at its lower end to the wellhead assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tide oscillations normally encountered in the marine environment.
This creates a problem because the portion of the stationary riser located within the hull opening of the oscillating vessel can contact and damage the vessel, unless it remains safely positioned within the hull opening. For this reason motion compensating equipment incorporated with the riser tensioning system is used to steady the riser within the hull opening, and usually takes the form of hydraulically actuated cable and sheave mechanisms connectably engaged between the upper riser elements and the vessel structure, and a flexible coupling located in the riser adjacent the vessel's hull. This equipment allows the vessel to heave and sway, without contacting the upper elements of the riser.
Directional positioning thrusters, in addition to the normal maneuvering system of the vessel, compensate for normal current and wind loading, and prevent riser separation due to the vessel being pushed away from the wellhead location.
All of these systems, however, can only prevent riser compressive failure, separation, or contact with the vessel during normal sea state conditions, or during normal operation of the dynamic positioning system.
The capacity of these systems may be exceeded by high winds and/or swells, which may occur during storms, or by failure of the vessel's dynamic positioning system which causes the vessel to "drive" off its normal position over the well. During either of these situations, measures need to be taken to prevent damage to the vessel and riser.
To avoid damage during a storm, the riser may be disassembled in sections and stowed on the floating vessel's deck, but the time required for this operation usually exceeds the warning time given by an oncoming storm, and certainly would not be sufficient in the event of an "instantaneous" positioning system failure, where the vessel drives uncontrollably off the well location.
An an alternative to disassembly of the riser, the riser may be disconnected from the wellhead assembly and thereby become suspended from the vessel. The vessel with the suspended riser then may remain in the vicinity of the wellhead assembly, or the vessel may attempt to tow the riser out of the path of an approaching storm. In either situation, once the riser's lower element is released from the wellhead assembly, the riser becomes a vertically oriented submerged vessel with its own oscillatory heave characteristics, or "bobbing" tendencies, typically different than those of the supporting vessel. When the riser, which may be under considerable tension from the tensioning system on the vessel, is released abruptly from the wellhead assembly, the riser may accelerate upward with the result that the upward movement of the riser often may exceed the displacement limits of the riser tensioning system. Also, when the vessel and disconnected riser heave upward, due to the vessel riding the crest of a wave, the riser may continue upward while the vessel is falling downward in a subsequent wave trough. This uncontrolled upward riser movement and subsequent downward movement through the center of the hull opening can exceed the allowable vertical movement and load capacity of the normal motion compensating and tensioning equipment, causing severe damage to the vessel and riser, with attendant risk to crew and vessel. A means is needed to prevent this uncontrolled riser movement.
As described in the two related copending applications both entitled "Drilling Riser Locking Apparatus and Method", filed Apr. 9, 1984, apparatus is disclosed which locks the upper end of the drilling riser to the vessel. This eliminates vertical and lateral movement of the riser relative to the vessel, obviating the above problem. The disclosed related apparatus is comprised of riser locking apparatus carried within the hull opening of the floating vessel adjacent the bottom of the vessel. The riser locking apparatus is carried at this lower elevation so that the angular displacement of the riser at its upper flexible coupling will not cause the riser, in its displaced position, to contact and damage the vessel's hull. The riser locking apparatus disclosed in both of these copending applications comprises a pair of movable beams that can be moved toward each other, at the closest point of travel engaging the upper elements of the riser. Locking these beams in their closed position effectively locks the riser's upper end to the vessel, thereby preventing the upper end of the riser from contacting the vessel.
In both of these copending applications, however, proper alignment of the riser with the locking beams in either the vertical or horizontal plane is necessary before the riser may be locked in position. For example, in copending application Ser. No. 597,994, vertical movement of the riser must be stopped before the movable beams can be closed. In copending application Ser. No. 597,995, the riser must be held in position in the center of the vessel's moon-pool before the riser can be raised between the movable beams and subsequently latched in place.
In both situations oscillation of the riser must be damped by devices other than the locking device, prior to the riser's being locked in place. Riser positioning means separate from the oscillation damping means must also be used. The operation of all of the above position and oscillation damping equipment requires close coordination and concentration by the vessel's crew, often during times of adverse sea conditions or in response to unexpected failure of the vessel's directional positioning system.
A device is needed therefore which combines the riser position and oscillation damping function in one device. Such a device is set forth in a copending application, Ser. No. 720,842, entitled "Drilling Riser Braking Apparatus and Method", filed Apr. 8, 1985. The riser braking apparatus disclosed in this third application comprises brake elements mounted at the end of beams which pivot within the hull opening of the vessel. As the beams pivot, the brake elements arc toward the center of the hull opening of the drilling vessel, contact the riser, centralize the riser within the hull opening, and arrest and lock the riser's upper elements to the vessel. This device therefore combines riser position and oscillation damping functions in one device. The proper operation of this device, however, requires that all of the brake elements interlock together after contact with the riser, so as to form a strong, unitized circular structure about the riser.
As may be imagined, every time the riser impacts the brake elements as they arc into position on the pivot beams, the likelihood of deformation of one or all of the pivot beams exists. Sufficient beam deformation may prevent interlock of the brake elements about the riser, which may prevent proper operation of this device.
A device is needed therefore that positions and arrests the riser centrally within the hull opening of the vessel, yet is not as susceptible to damage from riser impact loads during the riser arrestation process.